In conventional cellular communication systems, a UE (user equipment) has to communicate with another UE only through the relaying of base stations regardless of the distance of the two communicating UEs. FIG. 1 illustrates the conventional communication mode, wherein UE1 and UE2 interact with each other through the UTRAN consisting of base station transceiver (namely Node B) and RNC, and this communication mode is also called UP-UTRAN-DOWN mode. However, in some cases when the distance between two UEs who are camping in the same cell is very close, it can be a more reasonable way for them to communicate directly, rather than through the relaying of base stations. This method is the so-called peer-to-peer communication, abbr. as P2P.
FIG. 2 illustrates a P2P communication mode. As shown in FIG. 2, the dashed line represents signaling link, the solid line for data link, and the arrowhead for direction of information flow. Only signaling link exists between the UTRAN and the UE, while only data link exists between the two communicating UEs. Let's suppose only resource for maintaining basic communication is needed. If a direct link is taken as a radio resource unit (having fixed frequency, timeslot and spreading code), it can be easily inferred that P2P communication mode only needs two radio resource units to maintain basic communication. If additional signaling cost for management is ignored, P2P communication can save about 50% radio resource than conventional communication mode. Furthermore, the UTRAN still holds control over P2P communication, especially over how to use radio resources, so wireless network operators can easily charge for the radio resources used by P2P communication.
It is commonly accepted that a Time Division Duplex (TDD) air interface is a communication standard that offers a more flexible adaptation to different uplink and downlink traffic requirements. Among existing 3G systems based on TDD communication mode TD-SCDMA (Time Division—Synchronization Code Division Multiple Access) system is the most suitable system for the combination of P2P communication with conventional communication mode, because the same carrier frequency is applied in both uplink and downlink communications, which can simplify the RF (Radio Frequency) module of the UE.
A method and apparatus for establishing P2P communication in wireless communication networks, as described in the patent application entitled “A Method and Apparatus for Establishing P2P Communication in Wireless Communication Networks,” filed by KONINKLIJKE PHILIPS ELECTRONICS N.V. on Mar. 7, 2003, application Ser. No. 03119892.9, is suitable to any TDD CDMA communication system including TD-SCDMA systems, and incorporated herein as reference.
A method and apparatus for radio link establishment and maintenance with P2P communication in wireless communication networks, as described in the patent application entitled “A Method and Apparatus for Radio Link Establishment and Maintenance with P2P Communication in Wireless Communication Networks,” filed by KONINKLIJKE PHILIPS ELECTRONICS N.V. on Mar. 7, 2003, application Ser. No. 03119895.3, is suitable to any wireless communication system including TD-SCDMA systems, and incorporated herein by reference.
After establishing uplink synchronization with the UTRAN through the same random access procedure as existing TD-SCDMA systems, the UE can establish P2P direct link with another UE, in accordance with the method and apparatus as described in the application document whose application Ser. No. is 03119892.9, i.e.: allocate relevant dedicated resource for two P2P UEs. Then, direct link between the two UEs can be established and maintained in accordance with the method and apparatus as described in the application document whose application serial number is 03119895.3, so that the two UEs can receive and transmit P2P signals in the allocated timeslots respectively, and thus P2P communication between two UEs can be implemented.
In a TD-SCDMA system capable of employing P2P communication mode, DIRECT mode is introduced to describe the direct communication between two UEs, besides two other working modes—IDLE mode and CONNECT mode as defined in conventional TD-SCDMA system. The communication link in direct mode can be defined as FORWARD link (e.g.: the link from UE1 to UE2) and BACKWARD link (e.g.: the link from UE2 to UE1) identified by the information flow direction for one UE to send signals to the other UE or receive signals from the other UE. Because P2P communication mode is built in combination with existing TD-SCDMA systems, the UTRAN, the P2P communicating UEs and other conventional UEs allocated in the same timeslot can overhear the information transferred on the FORWARD link or BACKWARD link, i.e.: P2P communication changes the UP-UTRAN-DOWN mode in conventional TD-SCDMA systems. From the view of the UTRAN, even though the UEs have no connection with the UTRAN, the FORWARD link and BACKWARD link are associated with a certain uplink timeslot and/or downlink timeslot (the FORWARD link and BACKWARD link can correspond to different uplink timeslot and/or downlink timeslot depending on different resource allocation schemes). Hence, P2P communication will cause signal interference to conventional communication. Similarly, two P2P communicating UEs can also overhear the information transferred in the uplink timeslot or downlink timeslot associated with their FORWARD link or BACKWARD link during P2P communication. Therefore, when conventional links share the same timeslots with P2P link, conventional uplink or downlink communication will interfere with the P2P FORWARD link or BACKWARD link communication, which seriously deteriorates the performance of P2P-enabled TDD CDMA communication systems.
To improve the performance of P2P-enabled TDD CDMA communication systems, it's necessary to effectively reduce the signal interference caused by introducing P2P communication mode to the TD-SCDMA communication systems.
First of all, an analysis will go to the interference signals brought by introducing P2P communication mode in the following, and then how to reduce interference signals will be described. For simplicity, the timeslot in which one UE transmits signals to the other UE through the above FORWARD link or BACKWARD link is called transmit timeslot (Tx timeslot), while the timeslot in which the UE receives signals from another UE through the above FORWARD link or BACKWARD link is called receive timeslot (Rx timeslot), wherein the Tx timeslot and the Rx timeslot are respectively associated with an uplink timeslot and/or downlink timeslot in the sub-frame in conventional communication.
1. Interference Associated with Uplink Timeslot Between P2P Link and Conventional Link
FIG. 3 illustrates the interferences between P2P link and conventional link in P2P-enabled TD-SCDMA systems when the P2P link is associated with uplink timeslot. As shown in FIG. 3, it is assumed that UE1 and UE2 work in P2P mode and UE3 works in conventional mode, wherein UE1's Tx timeslot is associated with UE3's uplink timeslot, that is, UE1 and UE3 are allocated in the same uplink timeslot to transmit signals respectively to UE2 and the UTRAN. S1 is the information from UE1 to UE2 through direct link (taken as FORWARD link) and S2 is uplink information transmitted to the UTRAN via uplink from UE3, moreover, both S1 and S2 are associated with the same uplink timeslot but with different spreading codes.
In TD-SCDMA communication systems, one of the most important features is to maintain uplink synchronization, which means signals from different UEs should arrive at the UTRAN at the same time to guarantee the orthogonality of the spreading codes of signals from the main paths of different UEs. In this way, the system performance can be improved greatly by some advanced receiver algorithms and the computational complexity for the algorithms can be reduced greatly.
For conventional communication systems, the UTRAN is involved in every proceeding communication procedure as information source, destination or relayer, so it can monitor and control the UE' uplink transmitting time according to a specific traffic burst structure in CONNECT mode, and thus maintain uplink synchronization for each UE. But for P2P communication mode, the UTRAN is only involved in link establishment procedure and not involved in the P2P communication procedure afterwards. Therefore, during P2P communication, there is no dedicated channel between the UTRAN and the two P2P UEs, and the UTRAN can't adjust the synchronization advance of the two P2P UEs transmitting signals by using specific traffic burst to maintain uplink synchronization even if it can overhear and estimate the uplink synchronization shift of the two P2P UEs.
Referring to FIG. 3, when UE1 and UE3 transmit signals in the same uplink timeslot, the UTRAN can overhear information S1 transferred from UE1 to UE2 (to the UTRAN, S1 is considered as interference signal I1). But as described above, there is no dedicated channel between the UTRAN and UE1, so the UTRAN can't adjust UE1's transmission time by using the traffic burst in conventional communication mode even if it can overhear information S1 and estimate UE1's synchronization shift information, which means UE1 working in P2P mode may lose uplink synchronization with the UTRAN (UE3 working in conventional mode can maintain uplink synchronization with the UTRAN in conventional way). In another word, I1 and S2 are likely to arrive at the UTRAN unsynchronously, which will potentially impair uplink synchronization and thus degrade the system performance.
Similarly, when UE1 and UE3 transmit signals in the same allocated uplink timeslot, UE2 can also overhear signal S2 transferred from UE3 to the UTRAN (to UE2, S2 is considered as interference I2), and interference signal I2 will also produce impact on UE2 to receive S1, which may potentially impair the P2P communication quality.
2. Interference Associated with Downlink Timeslot Between P2P Link and Conventional Link
FIG. 4 illustrates the interferences between P2P link and conventional link in a P2P-enabled TD-SCDMA system when the P2P link is associated with downlink timeslot. As shown in FIG. 4, it is assumed that UE1 and UE2 work in P2P mode and UE3 works in conventional mode, wherein UE1's Rx timeslot is associated with UE3's downlink timeslot, that is, UE1 and UE3 are allocated in the same downlink timeslot to respectively receive signals from UE2 and the UTRAN. S3 is the P2P link information from UE2 to UE1 via direct link (taken as BACKWARD link) and S4 is downlink information from the UTRAN to UE3 via downlink, furthermore, both S3 and S4 are associated with the same downlink timeslot but with different spreading codes.
In FIG. 4, the downlink information S4 transmitted from the UTRAN to UE3 may produce interference to other UEs who share the same downlink timeslot with UE3 but use different spreading codes to receive signals. Such interference is called multi-access interference (MAI).
Referring to FIG. 4, when UE1 and UE3 are allocated in the same downlink timeslot to receive signals, UE1 can overhear information S4 transferred from the UTRAN to UE3 via downlink (to UE1, S4 is considered as interference signal I4), and generally the transmission power of signals from the UTRAN is relatively strong, so interference signal 14 is likely to impair the direct communication quality seriously.
Similarly, when UE1 and UE3 are allocated in the same downlink timeslot to receive signals, UE3 can also overhear information S3 transferred from UE2 to UE1 (to UE3, S3 is considered as interference signal I3, and meanwhile UE2 can be taken as the pseudo-UTRAN), and the interference signal I3 will impair the communication quality of UE3 near UE2 and other UEs in the same timeslot to receive signals as UE3.
3. Interference Between P2P Direct Link Pairs
FIG. 5 illustrates the interferences between two P2P direct link pairs in a P2P-enabled TD-SCDMA system, wherein a UE in one of the two P2P link pairs receives or transmits signals to the UE in another P2P link pair. Assume that UE1 and UE2 work in one P2P link pair while UE3 and UE4 in another P2P link pair.
Because P2P link pairs are symmetrical, signal S5 or S6 from UE1 to UE2 will become interference I5 or I6 to UE4 who is receiving signals from UE3 in associated timeslot. Obviously interference I5 or I6 may also greatly impair the direct communication quality.
As noted above, after P2P link is introduced in conventional TD-SCDMA systems, there exist 6 possible interference signals I1, I2, I3, I4, I5 and I6. Depending on whether the UTRAN is involved, the above 6 interference signals can be divided into two types. The first type includes interferences between the UEs, such as I2, I3, I5, and I6; and the second type includes interferences with UTRAN involved, such as I1 and I4.
To guarantee the communication quality of a P2P-enabled TD-SCDMA communication system, effective methods needs to be researched to cancel the above 6 interferences (it's better to achieve that without changing the physical layer structures of existing communication systems).
Regarding to interference signal I1 of the first type, two methods and apparatuses for canceling interference signal I1, are respectively elaborately described in the patent application document entitled “A Method and Apparatus for Uplink Synchronization Maintenance with P2P Communication in Wireless Communication Networks”, filed by KONINKLIJKE PHILIPS ELECTRONICS N.V. on Mar. 7, 2003, application Ser. No. 03119894.5, and another co-pending patent application document entitled “A Method and Apparatus for Uplink Synchronization Maintenance with P2P Communication in Wireless Communication Networks”, filed by KONINKLIJKE PHILIPS ELECTRONICS N.V. application Ser. No. 10/547,586and incorporated herein as reference.
As for interference signal I4 of the first type, a method and apparatus for canceling interference signal I4, is elaborately described in the patent application document entitled “A Method and Apparatus for Supporting P2P Communication in TDD CDMA Communication Systems”, filed by KONINKLIJKE PHILIPS ELECTRONICS N.V. on Apr. 14, 2003, application Ser. No. 03110415.0, and incorporated herein as reference.
As for interference signals I2, I3, I5 and I6 of the second type, collectively called Iaj, they can be reduced or cancelled by effectively limiting the radio range supported by P2P communication and adopting intelligent radio resource control scheme. Considering the limited P2P radio range, this invention proposes a scheme for canceling interference signal Iaj. This scheme can reduce interference signal Iaj by obtaining the mutual interference situation between a P2P UE and other UEs in the same cell (for example through position information) and allocating different timeslots to the P2P UE and its adjacent UEs.